fix: STT_ENABLE_DIARIZATION setting is wired through but silently discarded in both STT providers

## Bug Description

`application/core/settings.py` exposes `STT_ENABLE_DIARIZATION: bool = False` and it is correctly passed through the call chain — `attachments/routes.py:314` and `audio_parser.py:29` both pass `diarize=settings.STT_ENABLE_DIARIZATION` to `stt.transcribe()`. The `BaseSTT` abstract method also declares the parameter.

However both concrete implementations silently discard it:

```python
# openai_stt.py:29 AND faster_whisper_stt.py:42
_ = diarize # no-op
```

Users who set `STT_ENABLE_DIARIZATION=true` receive no diarization and no error or warning.

## Expected Behavior

When `diarize=True` is passed but the provider does not support it, a `logger.warning()` should be emitted so operators know the configuration has no effect.

## Proposed Fix

Replace `_ = diarize` in both providers with a conditional `logger.warning()` block.

Read openai_stt.py:29 and faster_whisper_stt.py:42, then trace the existing logger usage in each provider. Replace the discarded diarize value with a warning when diarize=True, and verify both providers warn that diarization is unsupported while preserving current transcription behavior.

Written by the indexing model from the issue text.